The prior-art documents relating to this invention are listed below.    Patent document 1: JP3074086 B (US5463084 A)    Patent document 2: JP2002-249578 A    Patent document 3: JP9-208674 A    Patent document 4: JP9-221625 A    Non-patent document 1: Journal of the Japanese Chemical Society, 1 (1993)    Non-patent document 2: Kogyo Zairyo, 49, (6), 53 (2001)    Non-patent document 3: Porisan no Kagaku, Kikan Kagaku Sosetsu No. 20, 1993, edited by the Japanese Chemical Society
Oxetane compounds have recently attracted attention as monomers capable of undergoing photoinitiated cationic polymerization or curing. For example, the patent document 1 describes a variety of oxetane derivatives and photocurable compositions. However, these compositions are not suitable for heat curing.
The non-patent document 2 describes the effectiveness of a quaternary onium salt such as tetraphenylphosphonium bromide for heat curing of oxetane compounds, However, the quaternary onium salt in question requires a temperature of 130° C. and a period of as long as 30 hours for the reaction and this is not practical. The patent document 2 describes a thermosetting composition involving the use of a metal salt such as a zinc carboxylate, but this composition is not satisfactory as it requires the conditions of 130° C. and 5 hours for the reaction.
The patent document 4 describes that a resin composition containing a sulfonium salt as a catalyst exhibits excellent low-temperature curability. However, the document further describes that it is essential for this resin composition to comprise a compound having both an oxetane functional group and a highly reactive epoxy group in the same molecule and gives no example of compounds having only an oxetane functional group. Moreover, the sulfonium salt, although highly reactive, contains antimony of high toxicity as a counter anion and is not desirable for practical use.
As described in the non-patent document 1, an organosilane and an aluminum chelate are used as a catalyst in the cationic polymerization of epoxy resins and the cured article is characterized by excellent electrical insulating properties as it is free from ionic impurities. The patent document 3 describes a curable oxetane composition comprising similarly an organosilane and an aluminum chelate as a catalyst. However, the incorporation of a highly reactive polyepoxide constitutes an essential condition and the document gives no example of the curing of an oxetane compound alone.